1. Field of the Invention
The present invention relates to an optical amplifier for repeating optical signals in an optical fiber system.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing the configuration of a conventional optical fiber repeating and transmitting system, as disclosed, for example, in the Patent Application Laid-Open No. 5-268167. FIG. 2 is a schematic diagram showing the detailed configuration of each of the optical amplifiers therein. In FIG. 1, reference numerals 1a to 1c denote a plurality of optical amplifiers provided in series, numerals 2a to 2d denote power feeding lines, 3a to 3d and 4a to 4d all denote optical fibers, 5 denotes a constant-current power supply for providing a constant current to the power feeding line 2a, and numerals 6a and 6b denote optical transmission terminal devices, respectively provided at the transmission end and the reception end of the transmission system.
As shown above, in the transmission lines 3a to 3d and 4a to 4d connecting the optical transmission terminal devices 6a and 6b, a plurality of optical amplifiers 1a to 1c are provided in series, and the power feeding to each of these optical amplifiers 1a to 1c is carried out normally by a constant-current power feeding method, in which a constant direct current is fed by a constant-current power supply provided in the above transmission end or the reception end of the transmission system by way of the power feeding lines 2a to 2d. 
In FIG. 2, numeral 1a denotes an optical amplifier, 2a and 2b denote power feeding lines, 3a, 3b, 4a and 4b denote optical fibers, numerals 11 and 12 denote optical repeater circuits for amplifying and repeating optical signals, 13 and 14 denote bypass circuits, which are connected respectively to the optical repeater circuits 11 and 12 in parallel for controlling the amount of current for driving the optical repeater circuits 11 and 12 (hereinafter referred to just as “drive current”), numeral 15 denotes a constant-current power supply connected to the optical repeater circuits 12 and the bypass circuit 14 in series, and numeral 16 denotes a constant-voltage diode connected in parallel to these repeater circuits 11 and 12, the bypass circuits 13 and 14, and the constant-current power supply 15.
Next, the operation of the conventional optical amplifier is now explained as follows.
The up-link optical signal input through the optical fiber 3a is amplified and repeated at the optical repeater circuit 12 and is output to the optical fiber 3b, whereas the down-link optical signal input through the optical fiber 4b is amplified and repeated at the optical repeater circuit 11 and is output to the optical fiber 4a. 
On the other hand, the optical repeater circuit 11 and the optical repeater circuit 12 are connected to each other in series, and in each of the optical repeater circuits 11 and 12, the bypass circuits 13 and 14 for allowing all or one part of the fed current to pass through are connected in parallel respectively. Due to this, the current input through the power feeding line 2a is branched to the optical repeater circuit 11 and the bypass circuit 13, and also branched to the optical repeater circuit 12 and the bypass circuit 14.
To the optical repeater circuit 11 and the bypass circuit 14, the constant-current power supply 15 is further connected in series, so as to limit the maximum current flowing into the optical repeater circuits 11 and 12. To each of these optical repeater circuits 11 and 12, the bypass circuits 13 and 14 and also the constant-current power supply 15, the constant-voltage diode 16 is connected in parallel. This constant-voltage diode 16 bypasses the excess current when an excessive amount of current is flown into the power feeding line 2a, thereby to implement a stable operation thereof.
As the conventional optical amplifier is constructed as explained above, the operating state of the optical repeater circuits 11 and 12 is controlled to keep constant even when the current fed to the optical amplifier 1a is fluctuated, and due to this, the operating state of these optical repeater circuits 11 and 12 cannot be changed from outside. Subsequently, there has been such a problem that when a malfunction occurs to one of the optical amplifiers due to the long-term use of a transmission system, the output level of other normal state optical amplifiers cannot be changed and so on.